Technology_ Term 1 & 2

Topics include: Structures; Properties of construction materials; Mechanical Systems; Pulley

Properties of materials

Properties of materials

• Hardness : ability of a stiff material to resist scratches, wear and tear or denting

• Flexibility : property of a material to bend and return to its original shape

• Corrosion : how well does it resist decay (rust etc.)

• Texture : smooth, bumpy, soft, hard, rough

• Comfort : what does it feel like to use

• Maintenance : how long will it last, what kind of upkeep is required

Members

Parts of a structure

Beams

Horizontal members

Columns

Vertical members

Cantilever

A beam or truss that is rigidly supported at one end, or in the middle, which has forces applied along the free arm or at the free arm

Structure of a stadium

• Frame structure

• Concrete slabs support seats

• Slabs are supported by a solid structure( concrete reinforced with steel)

• Frame rests on underground foundation( also supports floors inside stadium)

• Seating slabs and floor, made of reinforced concrete, are supported by their edges by the beams of the frame

• External walls (cladding materials) hang on the outside of the frame

Roof of a stadium

• Always supported from behind or above to avoid having columns that would block the spectators view

Static load

Loads that remain constant Do not change over time Include the force cause by the structure itself + forces caused by the weight on the structure

Dynamic load

Caused by forces which move across a structure

Corrosion

Destructive phenomenon which affects the appearance of an object May cause structural failure Caused by electrolytes- solutions that carry and electric charge

Natural/Artificial factors that cause corrosion

Perspiration High humidity Sunlight Rain, hail, flooding Animals

Vehicle exhaust/fumes Smokestack emissions Acid rain Vandalism Industrial chemicals

Ways to prevent corrosion

Dry out environment Place barrier between he metal and electrolyte (coat metal with other metal0 Place barrier between the electrolyte and electrode(electroplating)

Ergonomics

Deals with the way a product was designed to increase its productivity while reducing the operator's discomfort

Force

• Push or pull action on an object

• Have size + direction

• Can make a structure bend, stretch, compress or twist

• Act internally & externally on all structures

Dynamic forces

Forces acting on moving objects

Static forces

Forces acting on objects that are not moving

Struts

• Rigid members used where forces of compression act on a structure

• Used to stop a structure from collapsing

Ties

• Flexible members used where forces of tension act on a structure

• Used to stop a structure from collapsing

Different types of beams

L-beam I-beam T-beam U-beam Square beam

[LITUS]

Compression

When two equal and opposite forces decrease the length of an object

Torsion

When a turning force makes an object twist

Shear force

When opposing forces try to cut, tear or rip and object in two

Tension

When equal and opposite forces are applied to the ends of an object to increase its length

4 Types of construction materials

• Metals

• Non-metals

• Synthetic materials

• Natural materials

Gear

A wheel with teeth round a rim that is fixed to a shaft

Gear process

The first(driver) shaft rotates, the motion is transferred to the second(driven) shaft. The teeth link together so that the slide over one another as they turn in the opposite direction

Gear train

Number of gears connected together

• Used to increase/decrease speed

Velocity/Gear ratio

The difference between the big and small gear

• calculated using the number of teeth on the gear

Gearing up

The output speed is faster than the input speed (with gears)

What can gears do?

• Increase or decrease speed

• Change the direction of motion

• Multiply or increase turning forces

Idler gear

• Used to make the driver and the driven gear turn in the same direction

Crown & Pinion

• Type of bevel gear

• Can change the angle of movements by 90˚

Beveled gears

• Used to change the angle of movement by 90˚

• One gear turns vertically and the other horizontally

Worm and spur gear

• Used to reduce speed and increase force

• Used in model trains and cars

• Worm is DRIVEN

• Used to achieve high torque(turning force) and slow down movement

Rock and Pinion

• Changes rotary motion into linear motion

• Used in steering mechanism of car

Chain drive

• Links gears in a chain

• When gears are linked like this , they are called sprockets

Ratchet and Pawl

• Gear mechanism that can lock to prevent load from rolling backwards

• Used to allow movement in one direction only, while preventing movement in the opposite direction

• Used in mechanical jacks, winches in water wells

Cleat

• Safety device used to control movement in a pulley

• Used to clamp and hold a rope

• Secures rope by pinching it between/locking it around 2 pieces of metal or plastic

Piston

A disc attached to a rod that fits tightly inside a cylinder and that moves up and down the cylinder

Pascal's Principal

Pressure exerted on one part of a hydraulic system will be transferred equally, without loss, in all directions to other parts of the system

Pneumatic system

Controls mechanisms by using compressed gases

Advantage of a pneumatic system

• Cheaper than hydraulic systems

Disadvantages of a pneumatic system

• Kickback can occur

• Can be very dangerous, unpredictable

• Not suitable for precise controlled movements

Hydraulic system

Uses liquid under pressure to transmit force and motion in a machine

Advantages of a hydraulic system

• Pressure in the system is consistent

• Oil reduces friction

• Force is transferred directly and immediately

Disadvantage of a hydraulic system

• Leaks are expensive and messy

Hydraulic press

• Uses pressurized liquid to create a large amount of force

• Consists of cylinder and piston combination (hydraulic ram)

• Ram converts hydraulic power → mechanical power

Hydraulic press breakdown

• A pump forces oil at high pressures into the cylinder

• Valves control the flow of the pressurized liquid

• Pressure is transmitted by pressing a small piston into an enclosed tank of fluid

• The pressure is then transmitted through the fluid to a large piston = output force

• The force is multiplies because of the larger areas of the output piston

Hydraulic jack

A type of vehicle jack that uses oil under pressure to lift vehicles.

Components of a hydraulic jack

Pump - power source(hand powered by person pushing lever)

Reservoir- contains hydraulic fluid

Cylinder- hydraulic fluid is pumped into it(via on-way valve)

One-way valve- allows hydraulic fluid through the cylinder, does not allow it to pass back

Lifting arm- build-up of fluid causes pressure in the cylinder and pushes the arm to lift up

Systems diagram of a hydraulic jack

Input section: person operates lever

Process section: lever pumps hydraulic fluid from reservoir into the cylinder. Cylinder contains a one-way valve, which builds pressure

Output section: build-up of pressure pushes the arm to lift the car

Disc brake

• Most common type of brake system

• Car is slowed down by applying friction to a disc/rotor in the wheel of the car

Disc brake system break down

• Break pedal is a lever attached to a piston

• Piston is connected to a master cylinder filled with brake fluid

• Master cylinder is connected by piped filled with brake fluid to slave cylinders with pistons on all 4 wheels

• The pistons are attached to calipers (gripping mechanisms)

• Metal brake pads covered with friction material are attached to the end of each piston

• A cast iron/carbon composite brake disc is attached to the wheel of the car

Brake system process

• Driver presses brake pedal, the small input on the brake pedal exerts pressure on the brake fluid in the whole system

• Brake fluid from the master cylinder forces equal pressure through the whole system, into pistons on each wheel

• The pistons squeeze the two brake pads mounted on the calipers against the disc

• Friction between the pads and the disc slows, causing the wheels to slow

Rim brakes

Use brake blocks or brake shoes made of rubber/cork/leather to apply friction to the rim of a bicycle wheel

Advantages of rim brakes

Cheap Easy to maintain Mechanically simple A very effective control system

Bicycle brake system process

• When the rider operates the brake handle, a pull action on the cable causes the calipers to pull together

• This causes the brake blocks to apply pressure against the rim of the wheel

• The rubber creates friction, which converts kinetic energy → thermal energy, causing the bicycle to stop

Pulleys

-Help to lift heavy loads

• Often used in lifts and cranes

Compound pulley

A fixed pulley combined with a single/many pulleys

Single wheel fixed pulley

• Attached to a support above the load to be moved

• MA = 0 ; the force needed to pull with is the same as the weight of the object

Single wheel moveable pulley

• One end of the pulley rope is attached to a higher point, the pully itself is attached to the load being moved

Block and tackle

• A fixed pully combined with a moveable pulley

• Block = fixed pulley that is fixed to a structure

• Tackle = moveable pully

• Used to lift heavy loads using the mechanical advantage gained by using multiple pulleys to the system

• MA = 2; amount of force is halved

Fall

The number of times the rope supports the load

Mechanical advantage of pulleys

MA/Magnification of force is equal to the number of pulleys used. When distance is increased, force is decreased